3 先頭のビットが符号を表わすこと

sign.c

/*
 * sign.c --- MSB が符号を表わすことの確認
 */

#include <stdio.h>
#include <math.h>
#include "bdsd.h"

int main()
{
  double x;
  bdsd s;

  x = 1; double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");
  x *= -1; double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");

  x = 4.0 * atan(1.0); double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");
  x *= -1; double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");

  x = 0; double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");
  x *= -1; double_to_bin_digits(x, s);
  printf("x=%g\n", x); print_bdsd(s); printf("\n");

  return 0;
}

sign 実行結果

takebe% ./sign
x=1
0 01111111111 0000000000000000000000000000000000000000000000000000
x=-1
1 01111111111 0000000000000000000000000000000000000000000000000000
x=3.14159
0 10000000000 1001001000011111101101010100010001000010110100011000
x=-3.14159
1 10000000000 1001001000011111101101010100010001000010110100011000
x=0
0 00000000000 0000000000000000000000000000000000000000000000000000
x=0
1 00000000000 0000000000000000000000000000000000000000000000000000
takebe%

先頭のビットが符号を表わしていること、 また確かに $0$ にも二種類あることが分かる。